December

2011

 * December 11, 2011 - Yugata Kichigai is introduced
 * December 17, 2011 - Katsura Ryou is introduced
 * December 30, 2011 - HearMi is introduced

2009

 * December 17, 2009 - Larval Rin is introduced
 * December 21, 2009 - Kibiki Matsune is introduced

2007

 * December 00, 2007 - Vocaloids with Tsundere personalities appear. (see also - Tsundeloid)
 * December 11, 2007 - Kaiko, introduction date (assumption based on video upload)
 * December 14(?), 2007 - Tonarine Sai's debut
 * December 16, 2007 - Meito, introduction date (assumption based on video upload)
 * December 23, 2007 - Mouya Mete is introduced
 * December 23, 2007 - Amane Ann is introduced
 * December 24, 2007 - Yamine Aku is introduced

Misc

 * December 15, 1998 - Birthdate of Fuyu Hana
 * December 30, 1989 - Birthdate of Riza Sotone